EP3902799B1 - Complex of gadolinium and a chelating ligand derived of a diastereoisomerically enriched pcta and synthesis method - Google Patents

Description

The present invention relates to a new process for the synthesis of a complex of gadolinium and a chelating ligand derived from PCTA, which makes it possible to preferentially obtain the stereoisomers of said complex which have physicochemical properties which are particularly advantageous for applications as a contrast agent in the field of medical imaging, in particular for Magnetic Resonance Imaging.

Many contrast agents based on lanthanide chelates (paramagnetic metal), in particular gadolinium (Gd), are known, described for example in the patent US 4,647,447 . These products are often grouped under the term GBCA (Gadolinium-based Contrast Agent, contrast products based on gadolinium). Several products are on the market, among which are macrocyclic chelates, such as gadoterate meglumine based on DOTA (1,4,7,10-tetraazacyclododecane-N,N',N",N‴-tetraacetic acid), gadobutrol based on DO3A-butrol, gadoteridol based on HPDO3A, as well as linear chelates, in particular based on DTPA (diethylenetriaminepentaacetic acid) or DTPA-BMA (ligand of gadodiamide).

Other products, some of which are under development, represent a new generation of GBCAs. They are essentially complexes of macrocyclic chelates, such as the complexes of bicyclopolyazamacrocyclocarboxylic acid ( EP 0 438 206 ) or PCTA derivatives (i.e. comprising at least the chemical structure of 3,6,9,15-tetraazabicyclo[9,3,1]pentadeca-1(15),11,13 acid -triene-3,6,9-triacetic), as described in the document EP 1 931 673 .

Complexes of chelating ligands derived from PCTA described in document EP 1 931 673 have the particular advantage of being relatively easy to synthesize chemically, and, moreover, of having a relaxivity superior to other GBCAs (relaxivity r ₁ being able to go up to 11-12 mM ^-1 .s ^-1 in water) currently on the market, this relaxivity corresponding to the effectiveness of these products and therefore to their contrasting power.

In the body, the chelates (or complexes) of lanthanide - and in particular of gadolinium - are in a situation of chemical equilibrium (characterized by its thermodynamic constant K _therm ), which can lead to an undesired release of said lanthanide (see equation 1 below):

Chemical balance of complexation between the chelate or ligand (Ch) and the lanthanide (Ln) to lead to the Ch-Ln complex.

Since 2006, a pathology called NSF (Nephrogenic Systemic Fibrosis, systemic nephrogenic fibrosis or fibrogenic dermopathy), has been linked, at least in part, to the release of free gadolinium in the body. This disease led to an alert from the health authorities regarding gadoline contrast agents marketed for certain categories of patients.

Strategies have therefore been put in place to solve the complex problem of patient tolerance in a completely safe manner, and to limit, or even eliminate, the risk of unwanted lanthanide release after administration. This problem is all the more delicate to solve as the administration of contrast agents is often repeated, whether during diagnostic examinations, or for adjusting doses and monitoring the effectiveness of a therapeutic treatment.

In addition, since 2014, a possible cerebral deposit of gadolinium has been mentioned after repeated administration of gadolinium products, more particularly linear gadolinium chelates, such a deposit having not, or only slightly, been reported with macrocyclic gadolinium chelates, such as Dotarem ^® Consequently, various countries have decided either to withdraw most linear chelates from the market, or to drastically limit their indications for use, taking into account their stability deemed insufficient.

A strategy for limiting the risk of release of lanthanide in the organism thus consists in opting for complexes which are distinguished by the highest possible thermodynamic and/or kinetic stability. Indeed, the more the complex is stable, the more the quantity of lanthanide released over time will be limited.

However, the complexes of chelating ligands derived from PCTA comprising a structure of the pyclene type described in the document EP 1 931 673 , while having good kinetic stability, have, in general, a lower thermodynamic constant than that of the complexes of the other cyclene-derived macrocycles.

This is in particular the case of the complex of formula (II) represented below:

Indeed, as described in particular in the document WO 2014/174120 , the thermodynamic equilibrium constant corresponding to the formation reaction of the complex of formula (II), also called the stability constant, is 10 ^14.9 (ie log (K _therm )=14.9). For comparison, the stability constant of the gadolinium complex of 1, 4, 7, 10 tetra-azacyclododecane N, N', N", N'" tetra-acetic acid (DOTA-Gd), is 10 ^25.6 (ie log (K _therm ) = 25.6).

It should however be noted that the complex of formula (II) corresponds to several stereoisomers, in particular due to the presence of the three asymmetric carbon atoms located in position α on the side chains of the complex, with respect to the nitrogen atoms of the macrocycle on which said side chains are grafted. These three asymmetric carbons are marked with an asterisk (*) in formula (II) shown above.

Thus, the synthesis of the complex of formula (II) as described in the document EP 1 931 673 results in a mixture of stereoisomers.

The aminopropanediol groups of the side chains of the complex of formula (II) also contain an asymmetric carbon. Thus, the complex of formula (II) comprises a total of 6 asymmetric carbons, and therefore exists in the form of 64 configurational stereoisomers. However, in the rest of the description, the only source of stereoisomerism considered for a given side chain will be, for the sake of simplification, that corresponding to the asymmetric carbon bearing the carboxylate group, marked with an asterisk (*) in the formula (II ) shown above.

Insofar as each of these 3 asymmetric carbons can be of absolute configuration R or S, the complex of formula (II) exists in the form of 8 families of stereoisomers, called hereinafter II-RRR, II-SSS, II- RRS, II-SSR, II-RSS, II-SRR, II-RSR and II-SRS. More precisely, according to the usual nomenclature in stereochemistry, the complex of formula (II) exists in the form of 8 families of diastereoisomers.

The use of the term “family” is justified in that each of these families groups together several stereoisomers, in particular due to the presence of an asymmetric carbon within the aminopropanediol group, as mentioned above.

Nevertheless, insofar as, in the remainder of the description, the stereoisomerism linked to the asymmetric carbon of a given aminopropanediol group will not be considered, reference will be made indiscriminately to isomers, stereoisomers or even diastereoisomers II-RRR, II-SSS, II-RRS, II-SSR, II-RSS, II-SRR, II-RSR and II-SRS, without specifying that each corresponds to a family of stereoisomers.

The inventors have succeeded in separating and identifying by high performance liquid chromatography (HPLC, more commonly referred to by the English acronym HPLC) and by ultra high performance liquid phase chromatography (UHCP, more commonly referred to by the acronym English UHPLC) 4 masses or groups of isomers of the complex of formula (II) obtained according to the method of the prior art, corresponding to 4 different elution peaks characterized by their retention time on the chromatogram, which will be called iso1, iso2, iso3 and iso4 in the following description. By implementing the process described in the document EP 1 931 673 , the respective contents of the iso1, iso2, iso3 and iso4 groups in the mixture obtained are as follows: 20%, 20%, 40% and 20%.

They then discovered that these different groups of isomers had distinct physico-chemical properties, and determined that the group of isomers called iso4, which includes a mixture of the II-RRR and II-SSS isomers with the formulas (II-RRR ) and (II-SSS) represented below, proves to be the most advantageous as a contrast agent for medical imaging.

Thus, iso4 is distinguished, surprisingly, by a much greater thermodynamic stability than that of the mixture of diastereoisomers in the form of which the complex of formula (II) is obtained by implementing the process described in the document EP 1 931 673 . Indeed, its equilibrium thermodynamic constant K _{therm iso4} is equal to 10 ^18.7 (ie log (K _{therm iso4} ) = 18.7), this value having been determined by implementing the method described in Pierrard et al., Contrast Media Mol. Imaging, 2008, 3, 243-252 and Moreau et al., Dalton Trans., 2007, 1611-1620 .

Tableau 1 : cinétique de décomplexation des groupes d'isomères iso1 à iso4 Groupes d'isomères T_1/2 (pH 1,2 - 37 °C) Iso1 18 heures Iso2 6 heures Iso3 8 jours Iso4 27 jours Moreover, iso4 is the group of isomers which exhibits the best kinetic inertia (also called kinetic stability) among the four groups isolated by the inventors. Indeed, the inventors evaluated the kinetic inertia of the four groups of isomers by studying their kinetics of decomplexation in acidic aqueous solution (pH=1.2), at 37°C. The values of the half-life times (T _1/2 ) which were determined for each of the groups of isomers are indicated in table 1 below, the half-life time corresponding to the time after which 50% of the quantity of complex initially present has been dissociated, according to the following decomplexation reaction (equation 2) :

<u>Table 1</u>: decomplexation kinetics of groups of iso1 to iso4 isomers Isomer groups T _1/2 (pH 1.2 - 37°C) Iso1 18 hours Iso2 6 hours Iso3 8 days Iso4 27 days

By way of comparison, gadobutrol or gadoterate, macrocyclic gadolinium complexes, respectively show a kinetic inertia of 18 hours and 4 days under the same conditions, while linear gadolinium complexes such as gadodiamide or gadopentetate dissociate instantaneously.

In addition, iso4 is more stable from a chemical point of view than iso3 in particular. The amide functions of the complex of formula (II) are indeed capable of being hydrolyzed. The hydrolysis reaction of an amide function (equation 3) results in the formation of a dicoupled impurity, which is accompanied by the release of 3-amino-1,2-propanediol. The inventors have studied the kinetics of the hydrolysis reaction of the complex of formula (II) in aqueous solution at pH 13, and observed that the amide functions of iso4 are more stable with respect to hydrolysis than those of iso3.

With regard to the relaxivity of the different groups of isomers, that is to say their effectiveness as a contrast agent, the measurements carried out show a relatively equivalent contrasting power for the groups iso1, iso2 and iso4, and lower efficiency for iso3 (see Table 2). <u>Table</u> 2: relaxivity of groups of iso1 to iso4 isomers at 37°C Isomer groups r1 20 MHz (mM ^-1 .s ^-1 ) r1 60 MHz (mM ^-1 .s ^-1 ) Iso1 12.6 12.5 Iso2 13.3 12.9 Iso3 8.0 8.1 Iso4 12.9 13.0

The inventors have succeeded in developing a new process for preparing the complex of formula (II) making it possible to preferentially obtain the II-RRR and II-SSS diastereoisomers of said complex, which have particularly advantageous physicochemical properties. The process according to the invention comprises an isomeric enrichment step, by conversion of the least stable stereoisomers to the most stable stereoisomers, which, surprisingly, while being carried out on the intermediate hexaacid complex and not on the final complex , makes it possible to obtain the very majority of the most stable isomers of the complex of formula (II). The method according to the invention has been defined in the claims. Although the following description may for information include material going beyond the scope of the invention, this scope and therefore also the scope of protection remain limited to the subject matter defined by the claims.

The implementation of a process which makes it possible to obtain mainly the diastereoisomers of interest is incontestably advantageous compared to the alternative consisting in preparing the mixture of stereoisomers, in order then to attempt to separate the diastereoisomers according to the usual techniques and thus isolate the stereoisomers of interest. Indeed, in addition to the fact that it is easier to implement a process devoid of a step for separating diastereoisomers on an industrial scale, the absence of separation allows on the one hand a considerable saving of time, and, on the other hand, to improve the overall yield of the process, by limiting as much as possible the obtaining of the undesired diastereoisomers which would ultimately be eliminated. Furthermore, the usual separation techniques generally involve an abundant use of solvents, which, beyond the financial cost, is not desirable for environmental reasons. In addition, chromatography on silica in particular should be avoided, given the health risks inherent in occupational exposure to silica, classified as carcinogenic for humans (group 1) by the International Agency for Research against Cancer.

As indicated above, the process for preparing the complex of formula (II) developed by the inventors is based on a step of isomeric enrichment of the gadolinium complex of intermediate hexaacid of formula (I) represented below:

The complex of formula (I) corresponds to several stereoisomers, due to the presence of the three asymmetric carbon atoms located in the α position on the side chains of the complex, relative to the nitrogen atoms of the macrocycle on which the said side chains are grafted. . These three asymmetric carbons are marked with an asterisk (*) in formula (I) shown above.

Insofar as each of the 3 asymmetric carbons carrying a carboxylate function can be of absolute configuration R or S, the complex of formula (I) exists in the form of 8 stereoisomers, referred to below as I-RRR, I-SSS, I -RRS, I-SSR, I-RSS, I-SRR, I-RSR and I-SRS. More precisely, according to the usual nomenclature in stereochemistry, the complex of formula (I) exists in the form of 4 pairs of enantiomers, diastereoisomers between them.

The inventors succeeded in separating and identifying by high performance liquid phase chromatography (HPLC, more commonly referred to by the English acronym HPLC) and by ultra high performance liquid phase chromatography (UHPLC, more commonly designated by the English acronym UHPLC) 4 masses or groups of isomers of the complex of formula (I) obtained according to the process described in the document EP 1 931 673 , corresponding to 4 different elution peaks characterized by their retention time on the chromatogram, which will be called isoA, isoB, isoC and isoD in the remainder of the description.

IsoD crystallizes in water. X-ray diffraction analyzes enabled the inventors to determine the crystalline structure of this group of isomers, and thus to discover that it comprises the I-RRR and I-SSS diastereoisomers of the complex of formula (I), of formulas (I-RRR) and (I-SSS) shown below.

It should be noted that the I-RRR and I-SSS diastereomers of the complex of formula (I) are enantiomers of each other.

The isomeric enrichment step of the process of the invention aims to enrich the gadolinium complex with the intermediate hexaacid of formula (I) in isoD.

The synthesis of the complex of formula (II) notably involves a conversion of the carboxylic acid functions of the intermediate hexaacid complex of formula (I) into an amide function. This amidation reaction does not modify the absolute configuration of the three asymmetric carbon atoms of the complex of formula (I).

Thus, when the amidation reaction is carried out on the hexaacid complex of formula (I) enriched in isoD previously obtained, it makes it possible to obtain the complex of formula (II) enriched in iso4.

Gadolinium complex of hexaacid of formula (I)

constitué d'au moins 80 % d'un excès diastéréoisomérique comprenant un mélange d'isomères I-RRR et I-SSS de formules :

The process according to the invention therefore involves the hexaacid gadolinium complex of formula (I):

consisting of at least 80% of a diastereoisomeric excess comprising a mixture of I-RRR and I-SSS isomers of formulas:

The term "diastereoisomeric excess" is intended to denote, in the context of the present invention, and with regard to the complex of gadolinium of hexaacid of formula (I), the fact that said complex is mainly present in the form of an isomer or group of isomers chosen from among the diastereoisomers I-RRR, I-SSS, I-RRS, I-SSR, I-RSS, I-SRR, I-RSR and I-SRS. Said diastereoisomeric excess is expressed as a percentage, and corresponds to the quantity represented by the predominant isomer or group of isomers relative to the total quantity of the gadolinium complex of hexaacid of formula (I). It is understood that this percentage can be both molar and mass, insofar as isomers have, by definition, the same molar mass.

In a particular embodiment of the process according to the invention, the complex of formula (I) has at least 85%, in particular at least 90%, in particular at least 95%, preferably at least 97%, advantageously at least 98 %, more preferably at least 99% of the diastereoisomeric excess comprising the mixture of I-RRR and I-SSS isomers.

Preferably, said diastereoisomeric excess consists of at least 70%, in particular of at least 80%, advantageously of at least 90%, preferably of at least 95% of the mixture of I-RRR and I- SSS.

Advantageously, said diastereoisomeric excess consists of the mixture of I-RRR and I-SSS isomers.

The term “mixture of I-RRR and I-SSS isomers” also covers, by extension, the case where only one of the isomers, whether I-RRR or I-SSS, is present. However, the term “mixture of I-RRR and I-SSS isomers” preferably designates all the cases where each of the I-RRR and I-SSS isomers is present in a variable but non-zero amount.

In a preferred embodiment of the process according to the invention, the I-RRR and I-SSS isomers are present within said mixture in a ratio of between 65/35 and 35/65, in particular between 60/40 and 40/60 , in particular between 55/45 and 45/55. Advantageously, the mixture of I-RRR/I-SSS isomers is a racemic mixture (50/50).

More particularly, the diastereoisomeric excess as defined previously corresponds to peak 4 of the HPLC trace (that is to say the fourth peak in the order of elution and corresponding to isoD), characterized by a retention time comprised between 33.9 and 37.5 minutes, typically about 35.7 minutes, said trace being obtained by implementing the HPLC method described below.

By "HPLC trace" is meant, within the meaning of the present invention, the profile of the concentrations measured by the detector after passage and separation of a mixture of compounds (in this case isomers of a compound) on a phase stationary as a function of time for a given composition and eluent flow rate. The HPLC trace is made up of different peaks or peaks characteristic of the compound or mixture of compounds analyzed.

HPLC method:

- Symmetry ^® C18 column - 250 x 4.6 mm - 5 µm from Waters.
This is a reversed-phase HPLC column with spherical silica particles with C18 (octadecyl) grafting and whose silanols have been treated with capping agents (end-capped). It is also characterized by a length of 250 mm, an internal diameter of 4.6 mm, a grain size of 5 µm, a porosity of 100 Å and a carbon content of 19%.
Preferably, the stationary phase used must be compatible with the aqueous mobile phases.
- analysis conditions: Sample Aqueous solution of the complex of formula (I) at 10 mg/mL Column temperature 25°C Sample temperature Ambient temperature (20-25°C) Debit 1.0mL/min injection volume 20µL UV detection 200nm
- mobile phase gradient (volume): Time (mins) % acetonitrile (100%) % H ₂ SO ₄ (0.1% v/v aqueous solution) 0 1 99 10 5 95 40 10 90

Complex of formula (II)

constitué d'au moins 80 % d'un excès diastéréoisomérique comprenant un mélange d'isomères II-RRR et II-SSS de formules :

The process according to the invention makes it possible to obtain the complex of formula (II):

consisting of at least 80% of a diastereoisomeric excess comprising a mixture of II-RRR and II-SSS isomers of formulas:

By "diastereoisomeric excess" is meant, in the context of the present invention, and with regard to the complex of formula (II), the fact that said complex is mainly present in the form of an isomer or group of isomers chosen from among diastereoisomers II-RRR, II-SSS, II-RRS, II-SSR, II-RSS, II-SRR, II-RSR and II-SRS. Said diastereoisomeric excess is expressed as a percentage and corresponds to the quantity represented by the predominant isomer or group of isomers relative to the total quantity of the complex of formula (II). It is understood that this percentage can be both molar and mass, insofar as isomers have, by definition, the same molar mass.

In a particular embodiment, the complex of formula (II) obtained by the process according to the invention has at least 85%, in particular at least 90%, in particular at least 92%, preferably at least 94%, advantageously at least at least 97%, more preferably at least 99% of the diastereoisomeric excess comprising the mixture of II-RRR and II-SSS isomers.

Preferably, said diastereoisomeric excess consists of at least 70%, in particular of at least 80%, advantageously of at least 90%, preferably of at least 95% of the mixture of II-RRR and II-isomers. SSS.

Advantageously, said diastereoisomeric excess consists of the mixture of II-RRR and II-SSS isomers.

The term “mixture of II-RRR and II-SSS isomers” also covers, by extension, the case where only one of the isomers, whether II-RRR or II-SSS, is present. However, the term "mixture of II-RRR and II-SSS isomers" preferably designates the whole cases where each of the II-RRR and II-SSS isomers is present in a variable but non-zero amount.

In a preferred embodiment of the process according to the invention, the II-RRR and II-SSS isomers are present within said mixture in a ratio of between 65/35 and 35/65, in particular between 60/40 and 40/60 , in particular between 55/45 and 45/55. Advantageously, the II-RRR and II-SSS isomers are present within the mixture in a 50/50 ratio.

More particularly, the diastereoisomeric excess as defined above corresponds to peak 4 of the UHPLC trace (i.e. the fourth mass of isomers in the order of elution and corresponding to iso4), characterized by a time retention of between 6.0 and 6.6 minutes, typically about 6.3 minutes, said trace being obtained by implementing the UHPLC method described below.

By "UHPLC trace" is meant, within the meaning of the present invention, the profile of the concentrations measured by the detector after passage and separation of a mixture of compounds (in this case isomers of a compound) on a phase stationary as a function of time for a given composition and eluent flow rate. The UHPLC trace consists of different peaks or peaks characteristic of the compound or mixture of compounds analyzed.

UHPLC method :

• CORTECS ^® UPLC T3 column 150 x 2.1 mm - 1.6 µm from Waters.

This is a reverse phase UPLC column with spherical particles consisting of a core, preferably very hard, in silica surrounded by a porous silica with a trifunctional C18 (octadecyl) grafting, and whose silanols have been treated with styling agents (end-capped). It is also characterized by a length of 150 mm, an internal diameter of 2.1 mm, a grain size of 1.6 µm, a porosity of 120 Å and a carbon content of 4.7%.

Preferably, the stationary phase used must be compatible with the aqueous mobile phases.
- analysis conditions: Sample Aqueous solution of the complex of formula (II) at 2.0 mg/mL Column temperature 40°C Sample temperature Ambient temperature (20-25°C) Debit 0.3mL/min injection volume 1µL UV detection 200nm
- mobile phase gradient (% v /v): Time (mins) Acetonitrile (100%) H ₂ SO ₄ (0.0005% v/v aqueous solution) 0 1 99 3 5 95 12 10 90

In the process according to the invention, the complex of formula (II) is obtained by amidation from the complex of formula (I) as defined above and 3-amino-1,2-propanediol, in racemic form or enantiomerically pure, preferably in racemic form.

By “amidification” is meant, within the meaning of the present invention, the conversion reaction of a carboxylic acid function into an amide function by reaction with an amine function.

Such a reaction can in particular be carried out after activation of the carboxylic acid functions, as is detailed in the remainder of the description.

Process for the preparation of the complex of formula (II)

1. a) Complexation de l'hexaacide de formule (III) suivante
   
   avec du gadolinium pour obtenir le complexe de gadolinium d'hexaacide de formule (I) telle que définie précédemment,
2. b) Isomérisation par chauffage du complexe de gadolinium d'hexaacide de formule (I) dans une solution aqueuse à pH compris entre 2 et 4, pour obtenir un complexe diastéréoisomériquement enrichi constitué d'au moins 80 % d'un excès diastéréoisomérique comprenant un mélange des isomères I-RRR et I-SSS dudit complexe de gadolinium d'hexaacide de formule (I), et
3. c) Formation, à partir du complexe diastéréoisomériquement enrichi obtenu à l'étape b), du complexe de formule (II), par réaction avec le 3-amino-1,2-propanediol.

The present invention therefore relates to a process for preparing the complex of formula (II) comprising the following successive steps:

1. a) Complexation of the following hexaacid of formula (III)
   
   with gadolinium to obtain the hexaacid gadolinium complex of formula (I) as defined above,
2. b) Isomerization by heating of the hexaacid gadolinium complex of formula (I) in an aqueous solution at a pH of between 2 and 4, to obtain a diastereoisomerically enriched complex consisting of at least 80% of a diastereoisomeric excess comprising a mixture I-RRR and I-SSS isomers of said hexaacid gadolinium complex of formula (I), and
3. c) Formation, from the diastereoisomerically enriched complex obtained in step b), of the complex of formula (II), by reaction with 3-amino-1,2-propanediol.

In the present description, unless otherwise stated, the expressions “Gd”, “gadolinium” and “Gd ³⁺ ” are used interchangeably to designate the Gd ³⁺ ion. By extension, it can also be a source of free gadolinium, such as gadolinium chloride (GdCl ₃ ) or gadolinium oxide (Gd ₂ O ₃ ).

In the present invention, the term “free Gd” denotes the uncomplexed forms of gadolinium, and preferably available for complexation. This is typically the water-solubilized Gd ³⁺ ion. By extension, it can also be a source of free gadolinium, such as gadolinium chloride (GdCl ₃ ) or gadolinium oxide.

▪ Step a)

During this step, a complexation reaction takes place between the hexaacid of formula (III) and gadolinium, which makes it possible to obtain the gadolinium complex of hexaacid of formula (I) as defined above.

According to a particular embodiment, step a) comprises the reaction between the hexaacid of formula (III) and a source of free Gd in water.

In a preferred embodiment, the source of free Gd is GdCl ₃ or Gd ₂ O ₃ , preferably Gd ₂ O ₃ .

Preferably, the reagents used in step a), that is to say the source of gadolinium (typically gadolinium oxide), the hexaacid of formula (III) and water, are the purest. possible, especially with regard to metallic impurities.

Thus, the source of gadolinium will advantageously be gadolinium oxide, preferably with a purity greater than 99.99%, and even more preferably greater than 99.999%.

The water used in the process preferably comprises less than 50 ppm calcium, more preferably less than 20 ppm, and most preferably less than 15 ppm calcium. Generally, the water used in the process is deionized water, water for injection (water for injection) or purified water.

Advantageously, the quantities of the reagents (the hexaacid of formula (III) and the gadolinium) used during this stage a) correspond to, or are close to, stoichiometric proportions, as dictated by the equation-balance of the reaction of complexation taking place during this step.

By “close to stoichiometric proportions”, is meant that the difference between the molar proportions in which the reactants are introduced and the stoichiometric proportions is less than 15%, in particular less than 10%, preferably less than 8%.

The gadolinium can in particular be introduced in a slight excess with respect to the stoichiometric proportions. The ratio of the amount of material introduced in gadolinium to the amount of material introduced in hexaacid of formula (III) is then greater than 1, but typically less than 1.15, in particular less than 1.10, advantageously less than 1.08 . In other words, the amount of gadolinium introduced is greater than 1 equivalent (eq.), but typically less than 1.15 eq., in particular less than 1.10 eq., advantageously less than 1.08 eq., relative to the amount of hexaacid of formula (III) introduced, which for its part corresponds to 1 equivalent. In the preferred embodiment according to which the source of free gadolinium is Gd ₂ O ₃ , the quantity of Gd ₂ O ₃ introduced is then typically greater than 0.5 eq., but less than 0.575 eq., in particular less than 0, 55 eq., advantageously less than 0.54 eq., relative to the quantity of hexaacid of formula (III) introduced (1 eq.).

• a1) Préparation d'une solution aqueuse d'hexaacide de formule (III), et
• a2) Ajout, à la solution aqueuse obtenue à l'étape a1), d'une source de gadolinium libre.

According to a particular embodiment, step a) comprises the following successive steps:

• a1) Preparation of an aqueous solution of hexaacid of formula (III), and
• a2) Addition, to the aqueous solution obtained in step a1), of a source of free gadolinium.

In this embodiment, the content of hexaacid of formula (III) in the aqueous solution prepared during step a1) is typically between 10% and 60%, in particular between 15% and 45%, preferably between 20% and 35%, advantageously between 25% and 35%, even more advantageously between 25% and 30% by weight relative to the total weight of the aqueous solution.

Preferably, steps a) and b) are carried out according to a one-pot embodiment, that is to say in the same reactor and without an intermediate step of isolation or purification.

Thus, in this preferred embodiment, the hexaacid gadolinium complex of formula (I) formed during step a) is directly subjected to step b) of isomerization, without being isolated or purified, and in the same reactor as that used for step a).

▪ Step b)

The gadolinium complex of hexaacid of formula (I) formed by the complexation reaction between the hexaacid of formula (III) and gadolinium during step a) is initially obtained in the form of a mixture of diastereoisomers .

Step b) aims to enrich the mixture of diastereoisomers in the I-RRR and I-SSS isomers, to obtain the complex of gadolinium of hexaacid of formula (I) diastereoisomerically enriched consisting of at least 85%, in particular of at least 90%, in particular at least 95%, preferably at least 97%, advantageously at least 98%, more advantageously at least 99% of the diastereoisomeric excess comprising the mixture of isomers I -RRR and I-SSS.

Preferably, said diastereoisomeric excess consists of at least 70%, in particular of at least 80%, advantageously of at least 90%, preferably of at least 95% of the mixture of I-RRR and I- SSS.

Advantageously, said diastereoisomeric excess consists of the mixture of I-RRR and I-SSS isomers.

The inventors have in fact discovered that factors such as the pH and the temperature of the solution of gadolinium complex of hexaacid of formula (I) obtained at the end of step a) have an influence on the ratio in which the various isomers of the complex of formula (I) are present within the mixture of diastereoisomers. Over time, the mixture tends to be enriched in a group of isomers comprising the isomers which are, surprisingly, the most stable thermodynamically but also chemically, in this case the I-RRR and I-SSS isomers.

The term “mixture of I-RRR and I-SSS isomers” also covers, by extension, the case where only one of the isomers, whether I-RRR or I-SSS, is present. However, the term “mixture of I-RRR and I-SSS isomers” preferably designates all the cases where each of the I-RRR and I-SSS isomers is present in a variable but non-zero amount.

In a preferred embodiment, the I-RRR and I-SSS isomers are present within said mixture in a ratio of between 65/35 and 35/65, in particular between 60/40 and 40/60, in particular between 55/ 45 and 45/55. Advantageously, the mixture of I-RRR/I-SSS isomers is a racemic mixture (50/50).

Step b) of isomerization of the hexaacid gadolinium complex of formula (I) in an aqueous solution is typically carried out at a pH of between 2 and 4, in particular between 2 and 3, advantageously between 2.2 and 2, 8.

The pH is preferably adjusted with an acid, preferably an inorganic acid, such as hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid or phosphoric acid, for example with hydrochloric acid .

It is quite surprising that under such pH conditions, an enrichment of the mixture in particular isomers, in this case the I-RRR and I-SSS isomers, occurs, insofar as it is known in the art that gadolinium chelates are characterized by low kinetic inertia in an acid medium. Indeed, the higher the concentration of H ⁺ ions in the medium, the greater the probability that a proton will be transferred to one of the donor atoms of the ligand, thus causing the dissociation of the complex. Consequently, a person skilled in the art would have expected that placing the hexaacid gadolinium complex of formula (I) in an aqueous solution at a pH of between 2 and 4 would lead to the dissociation of said complex, and not not its isomerization to I-RRR and I-SSS.

It should be noted that the pH range recommended by the document EP 1 931 673 for the complexation of the hexaacid of formula (III), namely 5.0-6.5, does not make it possible to obtain the complex of formula (I) enriched in its I-RRR and I-SSS isomers.

Step b) is typically carried out at a temperature between 80°C and 130°C, in particular between 90°C and 125°C, preferably between 98°C and 122°C, advantageously between 100°C and 120°C. C, typically for a period of between 10h and 72h, in particular between 10h and 60h, advantageously between 12h and 48h.

Against all expectation, such temperature conditions, which, combined with the aforementioned pH conditions, should favor the instability of the gadolinium chelate, do not lead to its decomplexation or to the formation of any other impurity, but to its isomerization in I-RRR and I-SSS.

In a particular embodiment, the aqueous solution of step b) comprises acetic acid. Step b) is then advantageously carried out at a temperature of between 100°C and 120°C, in particular between 110°C and 118°C, typically for a period of between 12 h and 48 h, in particular between 20 h and 30 h, in particular between 24h and 26h.

The acetic acid is preferably added before heating the solution of gadolinium hexaacid complex of formula (I) obtained during step a) in an amount such that the acetic acid content is between 25% and 75%, in particular between 40% and 50% by mass relative to the mass of hexaacid of formula (III) used during step a).

When the aqueous solution is heated to a temperature advantageously between 100°C and 120°C, typically between 110°C and 118°C, acetic acid is added as the water evaporates, so as to maintain a constant volume of solution.

According to a preferred embodiment, at the end of step b), the diastereoisomerically enriched complex is isolated by crystallization, preferably by crystallization by seeding.

• b1) Isomérisation par chauffage du complexe de gadolinium d'hexaacide de formule (I) dans une solution aqueuse à pH compris entre 2 et 4, pour obtenir un complexe diastéréoisomériquement enrichi constitué d'au moins 80% de l'excès diastéréoisomérique comprenant le mélange des isomères I-RRR et I-SSS dudit complexe de gadolinium d'hexaacide de formule (I), et
• b2) Isolement par cristallisation dudit complexe diastéréoisomériquement enrichi de préférence par cristallisation par ensemencement.

In this embodiment, step b) comprises the following successive steps:

• b1) Isomerization by heating of the hexaacid gadolinium complex of formula (I) in an aqueous solution at pH between 2 and 4, to obtain a diastereoisomerically enriched complex consisting of at least 80% of the diastereoisomeric excess comprising the mixture I-RRR and I-SSS isomers of said hexaacid gadolinium complex of formula (I), and
• b2) Isolation by crystallization of said diastereoisomerically enriched complex, preferably by crystallization by seeding.

Step b2) of crystallization aims on the one hand to eliminate any impurities present in the aqueous solution, which may result from previous steps, so as to obtain a product of greater purity, discolored, in the form of crystals, and on the other hand to continue the diastereoisomeric enrichment of the gadolinium complex of hexaacid of formula (I), so as to obtain a diastereoisomeric excess comprising the mixture of the I-RRR and I-SSS isomers of said complex greater than that obtained at the result of step b1).

Indeed, the I-RRR and I-SSS isomers of the hexaacid complex of formula (I) crystallize in water. On the other hand, the hexaacid gadolinium complex of formula (I) not enriched in said isomers does not crystallize.

The fact that the I-RRR and I-SSS isomers, in which the complex tends to become enriched during step b) (and this against all expectation, given the conditions under which it is carried out), are the only isomers of the complex to crystallize in water constitutes a completely unexpected result. Isomerization and crystallization thus contribute synergistically to the enrichment in I-RRR and I-SSS isomers, and therefore to the overall efficiency of the process according to the invention.

Furthermore, it should be noted that the crystallization in water of the isomers of interest of the gadolinium complex of hexaacid of formula (I) makes it possible to avoid adding a solvent as described in example 7 of the document EP 1 931 673 , which involves a step of precipitation in ethanol of the trisodium salt of said complex.

Stage b2) is advantageously carried out at a temperature of between 10°C and 70°C, in particular between 30°C and 65°C, in particular between 35° and 60°C.

According to a variant, after lowering the temperature of the aqueous solution, so that the latter is within the ranges indicated above, the crystallization process is induced by seeding. "Crystallization by seed", also called "crystallization by seed", includes the introduction into the reactor in which the crystallization is carried out (also called crystallizer) of a known quantity of crystals, called "seed" or "starter". This reduces the crystallization time. Seed crystallization is well known to those skilled in the art.

In the process according to the invention, the seeding by use of a primer, in this case crystals of gadolinium complex of diastereoisomerically enriched hexaacid of formula (I) added to the aqueous solution of the diastereoisomerically enriched complex whose temperature has been previously lowered, makes it possible to obtain nucleation, and thus to initiate crystallization. The duration of the crystallization by seeding is advantageously between 2 h and 20 h, preferably between 6 h and 18 h, typically, it is 16 h.

The crystals of diastereoisomerically enriched hexaacid gadolinium complex of formula (I) are then typically isolated by filtration and drying, using any technique well known to those skilled in the art.

Advantageously, the degree of purity of the diastereoisomerically enriched gadolinium hexaacid complex of formula (I) isolated at the end of step b2) is greater than 95%, in particular greater than 98%, advantageously greater than 99%, said degree of purity being expressed as a mass percentage of the complex of formula (I) relative to the total mass obtained at the end of step b2).

In a particular embodiment, the diastereoisomerically enriched complex of step b) isolated by crystallization is again purified by recrystallization, to obtain a diastereoisomerically enriched and purified complex.

In this embodiment, step b) comprises, in addition to the successive steps b1) and b2) previously described, a step b3) of purification by recrystallization of the isolated diastereoisomerically enriched gadolinium hexaacid complex of formula (I).

Step b3) of recrystallization aims, like step b2) of crystallization, on the one hand, to obtain a product of greater purity, and, on the other hand, to continue the diastereoisomeric enrichment of the hexaacid gadolinium complex of formula (I), so as to obtain a diastereoisomeric excess comprising the mixture of the I-RRR and I-SSS isomers of said complex greater than that obtained at the end of step b2).

• mise en suspension du complexe de gadolinium d'hexaacide de formule (I) diastéréoisomériquement enrichi isolé lors de l'étape b2) en solution aqueuse, de préférence dans de l'eau,
• solubilisation dudit complexe par chauffage à une température avantageusement comprise entre 80°C et 120°C, par exemple à 100°C,
• recristallisation, de préférence par ensemencement, à une température avantageusement comprise entre 10°C et 90°C, notamment entre 20°C et 87°C, en particulier entre 55°C et 85°C, typiquement pendant une durée comprise entre 2h et 20h, notamment entre 6h et 18h, et
• isolement des cristaux de complexe de gadolinium d'hexaacide de formule (I) diastéréoisomériquement enrichi et purifié, par exemple par filtration et séchage.

Step b3) typically comprises the following successive sub-steps:

• suspending the diastereoisomerically enriched hexaacid gadolinium complex of formula (I) isolated during step b2) in aqueous solution, preferably in water,
• solubilization of said complex by heating to a temperature advantageously between 80° C. and 120° C., for example at 100° C.,
• recrystallization, preferably by seeding, at a temperature advantageously between 10°C and 90°C, in particular between 20°C and 87°C, in particular between 55°C and 85°C, typically for a period of between 2 hours and 8 p.m., in particular between 6 a.m. and 6 p.m., and
• isolation of the crystals of gadolinium complex of hexaacid of formula (I) diastereoisomerically enriched and purified, for example by filtration and drying.

The degree of purity of the purified diastereoisomerically enriched gadolinium hexaacid complex of formula (I) isolated at the end of step b3) is typically greater than 98%, in particular greater than 99%, advantageously greater than 99.5% , said degree of purity being expressed as a mass percentage of the complex of formula (I) relative to the total mass obtained at the end of step b2).

In another embodiment, the diastereoisomerically enriched complex of step b) is further enriched by selective decomplexation of the diastereoisomers of the complex of formula (I) other than the I-RRR and I-SSS diastereoisomers, i.e. by selective decomplexation of the diastereoisomers I-RSS, I-SRR, I-RSR, I-SRS, I-RRS and I-SSR.

In this embodiment, step b) comprises, in addition to the successive steps b1) and b2) previously described, a step b4) of selective decomplexation of the diastereoisomers of the complex of formula (I) other than the diastereoisomers I-RRR and I -SSS. In this variant, step b) can also comprise step b3) previously described, said step b3) being implemented between steps b2) and b4), or after step b4).

Step b4) of selective decomplexation aims to continue the diastereoisomeric enrichment of the complex of gadolinium with the hexaacid of formula (I), so as to obtain a diastereoisomeric excess comprising the mixture of the I-RRR and I-SSS isomers of said higher complex to that obtained at the end of step b2) or at the end of step b3), when the latter is implemented prior to step b4).

• mise en suspension du complexe de gadolinium d'hexaacide de formule (I) diastéréoisomériquement enrichi isolé lors de l'étape b2) ou lors de l'étape b3) dans de l'eau,
• ajout d'une base, par exemple de soude,
• chauffage à une température avantageusement comprise entre 30°C et 60°C, notamment entre 35°C et 55°C, par exemple à 40°C, typiquement pendant une durée comprise entre 2h et 20h, notamment entre 10h et 18h,
• refroidissement à une température avantageusement comprise entre 10°C et 30°C, par exemple à 30°C, et
• isolement du complexe de gadolinium d'hexaacide de formule (I) diastéréoisomériquement enrichi et purifié, par exemple par filtration et séchage.

Step b4) typically comprises the following successive sub-steps:

• suspending the diastereoisomerically enriched hexaacid gadolinium complex of formula (I) isolated during step b2) or during step b3) in water,
• addition of a base, for example soda,
• heating to a temperature advantageously between 30°C and 60°C, in particular between 35°C and 55°C, for example at 40°C, typically for a period of between 2h and 20h, in particular between 10h and 18h,
• cooling to a temperature advantageously between 10°C and 30°C, for example to 30°C, and
• isolation of the diastereoisomerically enriched and purified hexaacid gadolinium complex of formula (I), for example by filtration and drying.

Step b4) is made possible by the fact that the I-RRR and I-SSS isomers are the most stable in a basic medium. Such basic conditions favor the formation of gadolinium hydroxide, and consequently the decomplexation of the less stable isomers.

Thus, it should be noted that, surprisingly, the I-RRR and I-SSS isomers are more stable both in an acid medium, which allows step b1) of isomerization, and in a basic medium, which allows step b4) of selective decomplexation.

In a preferred embodiment, the diastereoisomerically enriched complex obtained at the end of step b) according to any one of the variants described above has at least 85%, in particular at least 90%, in particular at least 95 %, preferably at least 97%, advantageously at least 98%, more advantageously at least 99% of the diastereoisomeric excess comprising the mixture of I-RRR and I-SSS isomers.

Preferably, said diastereoisomeric excess consists of at least 70%, in particular of at least 80%, advantageously of at least 90%, preferably of at least 95% of the mixture of I-RRR and I- SSS.

Advantageously, said diastereoisomeric excess consists of the mixture of I-RRR and I-SSS isomers.

The term “mixture of I-RRR and I-SSS isomers” also covers, by extension, the case where only one of the isomers, whether I-RRR or I-SSS, is present. Nevertheless, the term “mixture of I-RRR and I-SSS isomers” preferably designates all the cases where each of the I-RRR and I-SSS isomers is present in a variable but non-zero amount.

In a preferred embodiment, the I-RRR and I-SSS isomers are present within said mixture in a ratio of between 65/35 and 35/65, in particular between 60/40 and 40/60, in particular between 55/ 45 and 45/55. Advantageously, the mixture of I-RRR/I-SSS isomers is a racemic mixture (50/50).

▪ Step c)

Step c) aims to form the complex of formula (II) from its precursor, the diastereoisomerically enriched hexaacid gadolinium complex of formula (I) obtained during step b).

During this step, the three carboxylic acid functions of the hexaacid complex of formula (I) carried by the carbon atoms located in the γ position on the side chains of the complex, relative to the nitrogen atoms of the macrocycle on which the said side chains, are converted into amide functions, by amidation reaction with 3-amino-1,2-propanediol, in racemic or enantiomerically pure form, preferably in racemic form.

This amidation reaction does not modify the absolute configuration of the three asymmetric carbon atoms located in the α position on the side chains, with respect to the nitrogen atoms of the macrocycle on which the said side chains are grafted. Consequently, step c) makes it possible to obtain the complex of formula (II) with a diastereoisomeric excess comprising a mixture of II-RRR and II-SSS isomers identical to the diastereoisomeric excess comprising a mixture of I-RRR isomers and I-SSS with which is obtained the diastereoisomerically enriched hexaacid gadolinium complex of formula (I) obtained at the end of step b), which is at least 80%.

In a preferred embodiment, the complex of formula (II) obtained at the end of step c) has at least 85%, in particular at least 90%, in particular at least 92%, preferably at least 94% , advantageously at least 97%, more advantageously at least 99% of the diastereoisomeric excess comprising the mixture of II-RRR and II-SSS isomers.

Preferably, said diastereoisomeric excess consists of at least 70%, in particular of at least 80%, advantageously of at least 90%, preferably of at least 95% of the mixture of II-RRR and II-isomers. SSS.

Advantageously, said diastereoisomeric excess consists of the mixture of II-RRR and II-SSS isomers.

The term “mixture of II-RRR and II-SSS isomers” also covers, by extension, the case where only one of the isomers, whether II-RRR or II-SSS, is present. However, the term “mixture of II-RRR and II-SSS isomers” preferably designates all the cases where each of the II-RRR and II-SSS isomers is present in a variable but non-zero amount.

In a preferred embodiment, the II-RRR and II-SSS isomers are present within said mixture in a ratio of between 65/35 and 35/65, in particular between 60/40 and 40/60, in particular between 55/ 45 and 45/55. Advantageously, the II-RRR and II-SSS isomers are present within the mixture in a 50/50 ratio.

The amidation reaction can be carried out according to any method well known to those skilled in the art, in particular in the presence of an agent which activates carboxylic acid functions and/or in acid catalysis.

It can in particular be carried out according to the methods described in the patent EP 1 931 673 , in particular in paragraph [0027] of this patent.

In a particular embodiment, step c) comprises the activation of the carboxylic acid functions (-COOH) of the hexaacid complex of formula (I) carried by the carbon atoms located in position γ on the side chains of the complex, by relative to the nitrogen atoms of the macrocycle on which the said side chains are grafted, in the form of derivative functions comprising a carbonyl group (C=O), which are such that the carbon atom of the carbonyl group is more electrophilic than the carbon atom of the carbonyl group of the carboxylic acid functions. Thus, according to this particular embodiment, said carboxylic acid functions can in particular be activated in the form of ester functions, acyl chlorides, acid anhydrides, or in any activated form capable of leading to an amide bond. The activated forms capable of leading to an amide bond are well known to those skilled in the art. profession and can for example be obtained by all the methods known in peptide chemistry to create a peptide bond. Examples of such methods are given in the publication Synthesis of peptides and peptidomimetics vol.E22a, p425-588, Houben-Weyl et al., Goodman Editor, Thieme-Stuttgart-New York (2004 ), and, among these examples, mention may be made in particular of methods for activating carboxylic acids via an azide (acyl azide), for example by the action of a reagent such as diphenylphosphoryl azide (commonly designated by the English acronym DPPA), the use of carbodiimides alone or in the presence of catalysts (for example N-hydroxysuccinimide and its derivatives), the use of a carbonyldiimidazole (1,1'-carbonyldiimidazole, CDI), the the use of phosphonium salts such as benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate (commonly referred to by the English acronym BOP) or even uroniums such as 2-(1H-benzotriazol-1-)hexafluorophosphate yl)-1,1,3,3-tetramethyluronium (commonly referred to by the acronym HBTU).

Preferably, step c) comprises the activation of the carboxylic acid functions (-COOH) mentioned above in the form of ester, acyl chloride or acid anhydride functions.

This embodiment is preferred to peptide coupling by activation of the carboxylic acid function using a coupling agent such as EDCI/HOBT as described in the document EP 1 931 673 . Indeed, such a coupling leads to the formation of an equivalent of 1-ethyl-3-[3-(dimethylamino)propyl]urea, which must be eliminated, in particular by chromatography on silica or by liquid/liquid extraction by adding a solvent . Independently of the complexity of the process generated by such an additional step, the implementation of such purification methods is not desirable, as discussed above. Moreover, the use of HOBT is in itself problematic, in that it is an explosive product.

By “ester function” is meant, within the meaning of the present invention, a —C(O)O— group. It may in particular be a —C(O)OR ₁ group, in which R ₁ corresponds to a (C ₁ -C ₆ )alkyl group.

By “(C ₁ -C ₆ )alkyl” group is meant, within the meaning of the present invention, a saturated hydrocarbon chain, linear or branched, comprising 1 to 6, preferably 1 to 4, carbon atoms. By way of example, mention may be made of the methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl or even hexyl groups.

By “acyl chloride function”, also called “acid chloride function”, is meant within the meaning of the present invention a —CO—CI group.

By “acid anhydride function” is meant, within the meaning of the present invention, a —CO—O—CO— group. It may in particular be a —CO—O—CO—R ₂ group, in which R ₂ corresponds to a (C ₁ -C ₆ )alkyl group.

The conversion reactions of a carboxylic acid function into an ester, acyl chloride or acid anhydride function are well known to those skilled in the art, who can implement them according to any usual method with which they are familiar.

The complex of formula (II) is then obtained by aminolysis of the carboxylic acid functions activated in the form of ester functions, acyl chlorides or acid anhydrides, in particular esters or acid anhydrides, preferably esters, by reaction with the 3-Amino-1,2-propanediol, in racemic or enantiomerically pure form, preferably in racemic form.

Preferably, the steps of activating the carboxylic acid functions and of aminolysis are carried out according to a one-pot embodiment, that is to say in the same reactor and without step intermediate for isolating or purifying the intermediate comprising the carboxylic acid functions activated in the form of esters, acyl chlorides or acid anhydrides, in particular esters or acid anhydrides, preferably esters.

• c1) formation d'un complexe activé de formule (VII),
  
  dans laquelle Y représente un atome de chlore, un groupement -OR₁ ou -O-C(O)-R₂, de préférence Y représente un groupement -OR₁ ou -O-C(O)-R₂, avec R₁ et R₂ correspondant, indépendamment l'un de l'autre, à un groupe (C₁-C₆)alkyle, et
• c2) aminolyse du complexe activé de formule (VII) avec le 3-amino-1,2-propanediol.

According to a particular embodiment, step c) comprises the following successive steps:

• c1) formation of an activated complex of formula (VII),
  
  in which Y represents a chlorine atom, an -OR ₁ or -OC(O)-R ₂ group, preferably Y represents an -OR ₁ or -OC(O)-R ₂ group, with R ₁ and R ₂ corresponding , independently of one another, to a (C ₁ -C ₆ )alkyl group, and
• c2) aminolysis of the activated complex of formula (VII) with 3-amino-1,2-propanediol.

As will clearly appear to those skilled in the art, the formation reaction of the activated complex of formula (VII) does not modify the absolute configuration of the three asymmetric carbon atoms located in position α on the side chains, with respect to the atoms of macrocycle nitrogen on which said side chains are grafted. Consequently, step c1) makes it possible to obtain the activated complex of formula (VII) with a diastereoisomeric excess comprising a mixture of the VII-RRR and VII-SSS isomers, of formulas (VII-RRR) and (VII-SSS) represented below, identical to the diastereoisomeric excess comprising a mixture of I-RRR and I-SSS isomers with which is obtained the complex of gadolinium of hexaacid of formula (I) diastereoisomerically enriched obtained at the end of step b), which is at least 80%.

In the case where Y represents a chlorine atom, step c1) is typically carried out by reaction between the gadolinium complex of hexaacid of formula (I) diastereoisomerically enriched obtained during step b) and thionyl chloride (SOCl ₂ ).

In the case where Y represents an -OC(O)-CH ₃ group, step c1) is typically carried out by reaction between the diastereoisomerically enriched gadolinium complex of hexaacid of formula (I) obtained during step b) and acetyl chloride.

In an advantageous embodiment, step c) comprises the activation of the carboxylic acid functions (-COOH) mentioned above in the form of ester functions.

• c1) formation d'un triester de formule (VIII),
  
  dans laquelle R₁ représente un groupe (C₁-C₆)alkyle, et
• c2) aminolyse du triester de formule (VIII) avec le 3-amino-1,2-propanediol.

According to this embodiment, step c) may more particularly comprise the following successive steps:

• c1) formation of a triester of formula (VIII),
  
  in which R ₁ represents a (C ₁ -C ₆ )alkyl group, and
• c2) aminolysis of the triester of formula (VIII) with 3-amino-1,2-propanediol.

Step c1) is typically carried out in the alcohol of formula R ₁ OH, which plays both the role of solvent and reagent, in the presence of an acid such as hydrochloric acid.

First, the hexaacid gadolinium complex of formula (I) and the alcohol R ₁ OH are loaded into the reactor. The reaction medium is then cooled to a temperature below 10°C, in particular below 5°C, typically at 0°C, and an acid solution of the alcohol R ₁ OH, typically of hydrochloric acid in R ₁ OH is then gradually added. The reaction medium is kept stirred at room temperature (that is to say at a temperature between 20 and 25° C.) for a period typically greater than 5 hours, preferably between 10 hours and 20 hours. The reaction medium is cooled at a temperature below 10° C., in particular between 0° C. and 5° C., prior to step c2).

Step c2) is also typically carried out in the alcohol of formula R ₁ OH, in the presence of an acid such as hydrochloric acid.

Thus, steps c1) and c2) can easily be implemented according to a one-pot embodiment. Advantageously, the triester of formula (VII) is not isolated between steps c1) and c2).

However, in order to promote the aminolysis reaction, during step c2), the alcohol of formula R ₁ OH is preferably removed by vacuum distillation.

By "vacuum distillation" is meant, within the meaning of the present invention, the distillation of a mixture carried out at a pressure of between 10 and 500 mbar, in particular between 10 and 350 mbar, preferably between 10 and 150 mbar, in particular between 50 and 100 mbar.

Similarly, in order to promote the aminolysis reaction, during step c2), 3-amino-1,2-propanediol is introduced in large excess. Typically, the amount of material of 3-amino-1,2-propanediol introduced is greater than 4 eq., in particular greater than 7 eq., advantageously greater than 10 eq., relative to the amount of material of gadolinium complex of hexaacid of formula (I) diastereoisomerically enriched initially introduced during step c), which corresponds to 1 equivalent.

Surprisingly, despite the acidic conditions typically employed during steps c1) and c2), which should increase the kinetic instability of the gadolinium complexes, no decomplexation or isomerization of the triester of formula (VIII ). The desired triamide is obtained with a very good conversion rate and the absolute configuration of the three asymmetric carbon atoms located in the α position on the side chains, with respect to the nitrogen atoms of the macrocycle, is preserved.

Furthermore, it should be noted that, in general, amidification reactions by direct reaction between an ester and an amine are very little described in the literature (see on this subject KC Nadimpally et al., Tetrahedron Letters, 2011, 52, 2579-2582 ).

• c1) formation d'un triester de méthyle de formule (IV),
  
  notamment par réaction dans du méthanol en présence d'un acide tel que l'acide chlorhydrique, et
• c2) aminolyse du triester de méthyle de formule (IV) avec le 3-amino-1,2-propanediol, notamment dans du méthanol en présence d'un acide tel que l'acide chlorhydrique.

In a preferred embodiment, step c) comprises the following successive steps:

• c1) formation of a methyl triester of formula (IV),
  
  in particular by reaction in methanol in the presence of an acid such as hydrochloric acid, and
• c2) aminolysis of the methyl triester of formula (IV) with 3-amino-1,2-propanediol, in particular in methanol in the presence of an acid such as hydrochloric acid.

Advantageously, the methyl triester of formula (IV) is not isolated between steps c1) and c2).

In a preferred embodiment, during step c2), the methanol is removed by distillation under vacuum, until a temperature typically greater than 55° C., in particular between 60° C. and 65° C., is reached, and the reaction medium is maintained at this temperature under vacuum for a period typically greater than 5 h, in particular between 10 h and 20 h, before being cooled to ambient temperature and diluted with water.

The present invention encompasses all the combinations of the particular, advantageous or preferred embodiments described above in connection with each step of the method.

constitué d'au moins 80 % d'un excès diastéréoisomérique comprenant un mélange d'isomères VIII-RRR et VIII-SSS de formules :

In a preferred embodiment, the process according to the invention therefore involves a triester gadolinium complex of formula (VIII):

consisting of at least 80% of a diastereoisomeric excess comprising a mixture of VIII-RRR and VIII-SSS isomers of formulas:

By "diastereoisomeric excess" is meant, in the context of the present invention, and with regard to the triester gadolinium complex of formula (VIII), the fact that said complex is predominantly present in the form of an isomer or group of isomers chosen from the VIII-RRR, VIII-SSS, VIII-RRS, VIII-SSR, VIII-RSS, VIII-SRR, VIII-RSR and VIII-SRS diastereoisomers. Said diastereoisomeric excess is expressed as a percentage, and corresponds to the quantity represented by the predominant isomer or group of isomers relative to the total quantity of the triester complex of formula (VIII). He is understood that this percentage can be both molar and mass, insofar as isomers have, by definition, the same molar mass.

In a particular embodiment of the process according to the invention, the triester gadolinium complex of formula (VIII) according to the invention has at least 85%, in particular at least 90%, in particular at least 95%, preferably at least least 97%, advantageously at least 98%, more advantageously at least 99% of the diastereoisomeric excess comprising the mixture of VIII-RRR and VIII-SSS isomers.

Preferably, said diastereoisomeric excess consists of at least 70%, in particular of at least 80%, advantageously of at least 90%, preferably of at least 95% of the mixture of isomers VIII-RRR and VIII- SSS.

Advantageously, said diastereoisomeric excess consists of the mixture of VIII-RRR and VIII-SSS isomers.

The term “mixture of VIII-RRR and VIII-SSS isomers” also covers the case where only one of the isomers, whether it be VIII-RRR or VIII-SSS, is present. However, the term “mixture of VIII-RRR and VIII-SSS isomers” preferably designates all the cases where each of the VIII-RRR and VIII-SSS isomers is present in a variable but non-zero quantity.

In a preferred embodiment of the process according to the invention, the VIII-RRR and VIII-SSS isomers are present within said mixture in a ratio of between 65/35 and 35/65, in particular between 60/40 and 40/60 , in particular between 55/45 and 45/55. Advantageously, the mixture of VIII-RRR/VIII-SSS isomers is a racemic mixture (50/50).

In a preferred embodiment of the process according to the invention, the triester gadolinium complex of formula (VIII) is a trimethyl gadolinium complex, that is to say a triester gadolinium complex of formula (VIII) in which R ₁ is a methyl group (CH ₃ ).

▪ Preparation of the hexaacid of formula (III)

The hexaacid of formula (III), which occurs during step a) of the process for preparing the complex of formula (II) according to the invention, can be prepared according to any method already known and in particular according to the methods described in the patent EP 1 931 673 .

avec un composé de formule R₃OOC-CHG_p-(CH₂)₂-COOR₄ (IX), dans laquelle :

• R₃ et R₄ représentent, indépendamment l'un de l'autre, un groupe (C₃-C₆)alkyle, notamment un groupe (C₄-C₆)alkyle tel qu'un groupement butyle, isobutyle, sec-butyle, tert-butyle, pentyle ou encore hexyle, et
• G_p représente un groupe partant tel qu'un groupement tosylate, triflate, ou un atome d'halogène, de préférence un atome de brome,

pour obtenir l'hexaester de formule (X)

suivie d'une étape d'hydrolyse, conduisant audit hexaacide de formule (III).However, according to a preferred embodiment, the hexaacid of formula (III) is obtained by alkylation of the pyclene of formula (V):

with a compound of formula R ₃ OOC-CHG _p -(CH ₂ ) ₂ -COOR ₄ (IX), in which:

• R ₃ and R ₄ represent, independently of one another, a (C ₃ -C ₆ )alkyl group, in particular a (C ₄ -C ₆ )alkyl group such as a butyl, isobutyl or sec-butyl group , tert-butyl, pentyl or even hexyl, and
• G _p represents a leaving group such as a tosylate or triflate group, or a halogen atom, preferably a bromine atom,

to obtain the hexaester of formula (X)

followed by a hydrolysis step, leading to said hexaacid of formula (III).

In a preferred embodiment, R ₃ and R ₄ are identical.

avec le 2-bromoglutarate de dibutyle, pour obtenir l'hexaester de butyle de formule (VI) :

suivie d'une étape d'hydrolyse, conduisant audit hexaacide de formule (III).According to an advantageous embodiment, the hexaacid of formula (III) is obtained by alkylation of the pyclene of formula (V):

with dibutyl 2-bromoglutarate, to obtain the butyl hexaester of formula (VI):

followed by a hydrolysis step, leading to said hexaacid of formula (III).

The dibutyl 2-bromoglutarate used is in racemic or enantiomerically pure form, preferably in racemic form.

The use of dibutyl 2-bromoglutarate is particularly advantageous, in comparison with that of ethyl 2-bromoglutarate described in the document EP 1 931 673 . In fact, commercial diethyl 2-bromoglutarate is a relatively unstable compound, which degrades over time and under the effect of temperature. More specifically, this ester tends to hydrolyze or cyclize and therefore to lose its bromine atom. Attempts to purify commercial diethyl 2-bromoglutarate, or to develop new synthetic routes allowing it to be obtained with improved purity, and thus prevent its degradation, have been unsuccessful.

The alkylation reaction is typically carried out in a polar solvent, preferably in water, in particular in deionized water, advantageously in the presence of a base such as potassium or sodium carbonate.

The use of water is preferred in particular to that of acetonitrile, described in the document EP 1 931 673 , for obvious reasons.

The reaction is advantageously carried out at a temperature between 40° C. and 80° C., typically between 50° C. and 70° C., in particular between 55° C. and 60° C., for a period of between 5 h and 20 h, in particular between 8 a.m. and 3 p.m.

The hydrolysis step is advantageously carried out in the presence of an acid or a base, advantageously a base such as sodium hydroxide. The hydrolysis solvent can be water, an alcohol such as ethanol, or a water/alcohol mixture. This step is advantageously carried out at a temperature of between 40°C and 80°C, typically between 40°C and 70°C, in particular between 50°C and 60°C, typically for a period of between 10h and 30h, in particular between 3 p.m. and 10 p.m.

In a preferred embodiment, the process according to the invention therefore involves the butyl hexaester of formula (VI):

Indeed, this hexaester is distinguished by a markedly improved stability compared to esters having a shorter alkyl chain, in particular compared to the ethyl hexaester described in the document EP 1 931 673 .

FIGURES:

Figure 1 : degradation under basic conditions of the groups of iso1 to iso4 isomers of the complex of formula (II), expressed as surface percentage of a given group of isomers over time.

EXAMPLES

The examples given below are presented by way of non-limiting illustration of the invention.

Separation of the isoA, isoB, isoC and isoD isomer groups of the hexaacid gadolinium complex of formula (I) by HPLC

An HPLC apparatus consisting of a pumping system, an injector, a chromatographic column, a UV spectrophotometric detector and a control and data processing station is used. The chromatographic column used is a C18 column - 250×4.6 mm - 5 μm (Symmetry® ^range , from Waters).

- Moving phase :

Route A: 100% acetonitrile and Route B: aqueous solution of H2SO4 (96%) at 0.1% v/v

- Preparation of test solutions :

Solution of the hexaacid gadolinium complex of formula (I) at 10 mg/mL in purified water.
- Analysis conditions : Column temperature 25°C Sample temperature Ambient temperature (20-25°C) Debit 1.0ml/min injection volume 20µl UV detection 200nm Analysis time 60 mins
- Gradient : Time % Acne % H ₂ SO ₄ 0.1% 0 1 99 10 5 95 40 10 90 50 25 75 55 1 99 60 1 99 %Acn: % v/v of acetonitrile in the mobile phase
% H ₂ SO ₄ 0.1%: % v/v of the solution of H ₂ SO ₄ at 0.1% v/v in the mobile phase

4 main peaks are obtained. Peak 4 of the HPLC trace, namely the isoD, corresponds to a retention time of 35.7 minutes.

Separation of the isoA, isoB, isoC and isoD isomer groups of the hexaacid gadolinium complex of formula (I) by UHPLC

A UHPLC apparatus consisting of a pumping system, an injector, a chromatographic column, a UV detector and a data station is used. The chromatographic column used is a 150×2.1 mm −1.8 μm UHPLC column (ACQUITY UPLC HSS T3 column from Waters). It is a reversed-phase UPLC column with spherical particles made of silica with a trifunctional C18 (octadecyl) grafting, and whose silanols have been treated with capping agents (end-capped). It is further characterized by a length of 150 mm, an internal diameter of 2.1 mm, a particle size of 1.8 µm, a porosity of 100 Å and a carbon content of 11%.

Preferably, the stationary phase used must be compatible with the aqueous mobile phases.
- Moving phase :
Route A: 100% acetonitrile and Route B: aqueous solution of H ₂ SO ₄ (96%) at 0.1% v/v
- Preparation of test solutions :
Solution of the hexaacid gadolinium complex of formula (I) at 0.8 mg/mL in purified water
- Analysis conditions : Column temperature 35°C Sample temperature Ambient temperature (20-25°C) Debit 0.4mL/min injection volume 10µl UV detection 200nm Analysis time 32 mins
- Gradient : Time % Acne % H ₂ SO ₄ 0.1% 0 1 99 14 8 92 20 11 89 25 25 75 27 1 99 32 1 99

4 main peaks are obtained. Peak 4 of the UHPLC trace, namely the isoD, corresponds to a retention time of 17.4 minutes.

Separation of the iso1, iso2, iso3 and iso4 isomer groups of the complex of formula (II) by UHPLC

A UHPLC apparatus consisting of a pumping system, an injector, a chromatographic column, a UV detector and a data station is used. The chromatographic column used is a 150×2.1 mm −1.6 μm UHPLC column ( ^CORTECS® UPLC T3 column from Waters).
- Moving phase :
Route A: 100% acetonitrile and Route B: aqueous solution of H ₂ SO ₄ (96%) at 0.0005% v/v
- Preparation of test solutions :
Solution of the complex of formula (II) at 2 mg/mL in purified water
- Analysis conditions : Column temperature 40°C Sample temperature Ambient temperature (20-25°C) Debit 0.3mL/min injection volume 1µl UV detection 200nm Analysis time 20 mins
- Gradient : Time % Acne % _{H2SO4 0.0005} % 0 1 99 3 5 95 12 10 90 15 25 75 16 1 99 20 1 99

4 main peaks are obtained. Peak 4 of the UHPLC trace, namely iso4, corresponds to a retention time of 6.3 minutes.

Relaxivity measures

The relaxation times T ₁ and T ₂ were determined by standard procedures on a Minispec ^® mq20 apparatus (Bruker) at 20 MHz (0.47 T), at 60 MHz (1.41 T) and 37°C. The longitudinal relaxation time T ₁ is measured using an Inversion Recovery sequence and the transverse relaxation time T ₂ is measured by a CPMG (Carr-Purcell-Meiboom-Gill) technique.

The relaxation rates R ₁ (= 1/T ₁ ) and R ₂ (= 1/T ₂ ) were calculated for different concentrations of total metal (varying from 0.5 x 10 ^-3 to 5 x 10 ^-3 mol/ L) in aqueous solution at 37°C. The correlation between R ₁ or R ₂ as a function of the concentration is linear, and the slope represents the relaxivity r ₁ (R ₁ /C) or r ₂ (R ₂ /C) expressed in (1 / second) x (1/ mMole/L) or (mM ^-1 .s ^-1 ).

Measurement of the kinetic inertia of the groups of isomers of the complex of formula (II) in an acid medium

The dissociation of the gadolinium complexes present in the 4 masses of iso1 to iso4 isomers (C = 8.10-6 M) is studied at 37°C, pH 1.2, in a hydrochloric acid solution under kinetic conditions of pseudo first order without ionic strength control by following the release of gadolinium into solution. The quantity of free gadolinium was determined by spectrometry at 654 nm after adding a solution of Arsenazo III (C=5.3×10 −4 M).

The half-life times (T _1/2 ) which were determined for each of the groups of isomers are given in the table below: Isomer groups T _1/2 (pH 1.2 - 37°C) Iso1 18 hours Iso2 6 hours Iso3 8 days Iso4 27 days

Study of degradation under basic conditions of the groups of isomers of the complex of formula (II)

The complex of formula (II) will be called PA in the remainder of this example.

The degradation kinetics of the blocks of iso1 to iso4 isomers, designated by the generic term isoX, are evaluated by measuring the HPLC purity and by monitoring the area of each block of isomers over time. The quantities measured are thus: - P _HPLC (time), and $$\frac{{\mathit{Area}}_{\mathit{IsoX}}\left(\mathrm{you}\right)}{{\mathit{Area}}_{\mathit{IsoX}}\left({\mathrm{you}}_0\right)}$$

The chosen degradation conditions are as follows: [PA]=1 mM in 0.1 N sodium hydroxide. Under these dilution conditions, the impact of the degradation of the PA on the experimental medium is low. The degradation products do not modify the pH of the medium, this parameter being critical in the study of degradation kinetics. This is confirmed experimentally by measuring the initial pH and at the end of degradation (72 hours, 37°C): PA solution pH of _0.1N NaOH (T0) 12.9 pH (after 72 hours at 37°C) 12.8

• peser environ 0,05 g de chaque produit qsp 10 mL de l'eau mQ, pour obtenir une solution A telle que [PA]_A = 5 mM,
• dilution : 2 mL de solution A qsp 10 mL NaOH (0,1 N), pour obtenir une solution B telle que [PA]_B = 1 mM et [NaOH] = 0.08 M,
• aliquote des solutions dans des flacons HPLC, et
• incubation des flacons HPLC contenant les solutions de PA dans NaOH à la température de l'étude (37°C).

The solution preparation method is described below:

• weigh approximately 0.05 g of each product qsp 10 mL of mQ water, to obtain a solution A such that [PA] _A = 5 mM,
• dilution: 2 mL of solution A qs 10 mL NaOH (0.1 N), to obtain a solution B such that [PA] _B = 1 mM and [NaOH] = 0.08 M,
• aliquot solutions into HPLC vials, and
• incubation of the HPLC flasks containing the solutions of PA in NaOH at the temperature of the study (37° C.).

For each point, an aliquot is taken and analyzed by HPLC without diluting the sample (ammonium acetate method).

The results obtained are reported in Figure 1 .

Preparation of the butyl hexaester of formula (VI)

In a reactor, 184 kg (570 moles) of dibutyl 2-bromoglutarate and 89 kg (644 moles) of potassium carbonate are mixed and heated to 55-60°C. An aqueous solution of 29.4 kg (143 moles) of pyclene in 24 kg of water is added to the previous preparation. The reaction mixture is maintained at 55-60° C. and then heated under reflux for about ten hours. After reaction, the medium is cooled, diluted with 155 kg of toluene then washed with 300 liters of water. The butyl hexaester is extracted in the aqueous phase with 175 kg (1340 moles) of phosphoric acid (75%). It is then washed 3 times with 150 kg of toluene. The butyl hexaester is reextracted in the toluene phase by dilution with 145 kg of toluene and 165 kg of water followed by basification with 30% (m/m) soda to reach a pH of 5-5.5. The lower aqueous phase is removed. The butyl hexaester is obtained by concentration to dryness under vacuum at 60° C. with a yield of approximately 85%.

Preparation of the hexaacid of formula (III)

In a reactor, 113 kg (121 moles) of butyl hexaester are loaded as well as 8 kg of ethanol. The medium is brought to 55+/-5° C. then 161 kg (1207.5 moles) of 30% (m/m) sodium hydroxide are poured in over 3 hours. The reaction mixture is maintained at this temperature for about twenty hours. The butanol is then removed by decantation from the reaction medium. The hexaacid of formula (III) obtained in the form of sodium salt is diluted with water to obtain an aqueous solution of about 10% (m/m). This solution is treated on an acid cationic resin. The hexaacid of formula (III) in aqueous solution is obtained with a yield of approximately 90% and a purity of 95%.

Preparation of the hexaacid gadolinium complex of formula (I) ▪ Experimental Protocol • Complexation and isomerization - Without acetic acid

418 kg (117 kg of pure hexaacid of formula (III)/196 mol) of an aqueous solution of hexaacid of formula (III) at 28% by weight are loaded into a reactor. The pH of the solution is adjusted to 2.7 by adding hydrochloric acid, then 37 kg (103.2 moles) of gadolinium oxide are added. The reaction medium is heated at 100-102° C. for 48 hours to achieve the expected isomeric distribution of the hexaacid of formula (III).

- With acetic acid

Gadolinium oxide (0.525 molar eq.) is suspended in a solution of hexaacid of formula (III) at 28.1% by weight.

The 99-100% acetic acid (50% by mass/pure hexaacid of formula (III)) is poured onto the medium at room temperature.

The medium is heated to reflux and then distilled to 113° C. by mass, reloading the medium with acetic acid as the water is eliminated. Arrived at 113° C., a sufficient quantity of acetic acid is added in order to arrive at the starting volume.

The medium is maintained at 113° C. overnight.

• Crystallization, Recrystallization - Crystallization

The hexaacid gadolinium complex of formula (I) in solution is cooled to 40° C., the primer is added, the mixture is left in contact for at least 2 hours. It is then isolated by filtration at 40° C. and washed with osmosed water.

- Recrystallization

180 kg of the hexaacid gadolinium complex of formula (I) obtained above (dry extract at approximately 72%) are suspended in 390 kg of water. The medium is heated to 100° C. to dissolve the product, then cooled to 80° C. to be primed by adding a little primer. After cooling to ambient temperature, the hexaacid gadolinium complex of formula (I) is isolated by filtration and drying.

• Selective decomplexation

The dry product is charged to the reactor with osmosis water at 20°C. The mass of water added is equal to twice the mass of gadolinium hexaacid complex of theoretical formula (I). 30.5% (m/m) sodium hydroxide (6.5 eq) is poured onto the medium at 20°C. The medium is left in contact at 50° C. at the end of the addition of NaOH for 16:00. The medium is cooled to 25° C. and the product filtered through a bed of Clarcel.

▪ Diastereoisomeric excess content comprising a mixture of I-RRR and I-SSS diastereoisomers

The ratio in which the different isomers of the complex of formula (I) are present within the mixture of diastereoisomers depends on the conditions under which the complexation and isomerization steps are carried out, as shown in Table 3 below. <u>Table 3</u>: content of the I-RRR and I-SSS mixture as a function of the complexation / isomerization conditions pH Temperature Content of hexaacid of formula (III) Duration Diastereoisomeric excess comprising a mixture of I-RRR and I-SSS 5.7 80°C 40% 3h 19% 3.5 90°C 50% 10am 49% 3.0 101°C 40% 10am 68% 2.7 101°C 28% 48h 98.04%

The additional stages of recrystallization and selective decomplexation make it possible to increase the diastereoisomeric excess in the I-RRR and I-SSS mixture (see table 4). <u>Table 4:</u> diastereoisomeric excess content comprising a mixture of I-RRR and I-SSS after crystallization / recrystallization / selective decomplexation After the ^1st crystallization After recrystallization After selective decomplexation Diastereoisomeric excess comprising a mixture of I-RRR and I-SSS 98.04% 99.12% 99.75%

Preparation of the complex of formula (II)

In a reactor, 90 kg (119 moles) of the hexaacid complex of formula (I) and 650 kg of methanol are loaded. The mixture is cooled to approximately 0°C then 111 kg (252 moles) of a solution of methanolic hydrochloric acid (8.25% HCl in methanol) are poured in while maintaining the temperature at 0°C. The reaction medium is brought to room temperature and then kept under stirring for 16 hours. After cooling to 0-5°C, 120 kg (1319 moles) of 3-amino-1,2-propanediol are added. The reaction medium is then heated by distilling the methanol under vacuum until a temperature of 60-65°C is reached. The concentrate is maintained for 16 hours at this temperature under vacuum. At the end of contact, the medium is diluted with 607 kg of water while cooling to ambient temperature. The solution of the crude complex of formula (II) is neutralized with 20% (m/m) hydrochloric acid. 978.6 kg of solution are thus obtained, with a concentration of 10.3%, representing 101 kg of material. The yield obtained is 86.5%.

Isomer conversion tests from the complexes of formula (II)

• isoA + APD(R) et isoA + APD(S),
• isoB + APD(R) et isoB + APD(S),
• isoC + APD(R) et isoC + APD(S), et
• isoD + APD(R) et isoD + APD(S).

The isomers of the complex of formula (II) were synthesized from the isoA, isoB, isoC and isoD isomer groups of the hexaacid complex of formula (I) isolated by preparative HPLC. The 4 groups of isomers were isolated and then amidified with 3-amino-1,2-propanediol (APD) R and S. 8 isomers are thus obtained:

• isoA + APD(R) and isoA + APD(S),
• isoB + APD(R) and isoB + APD(S),
• isoC + APD(R) and isoC + APD(S), and
• isoD + APD(R) and isoD + APD(S).

Each of these isomers was placed under the conditions allowing the isomerization of the gadolinium complex of hexaacid of formula (I).

Thus, an HCl solution at pH 3 is prepared by diluting 1 mL of 1N HCl in 1 liter of water. The isomers are added at a concentration of 1 mM in the HCl solution at pH 3. 10 mg of powder are dissolved in 10 mL of this solution. The 8 solutions obtained are heated to 100° C. and then analyzed at T ₀ and at T ₀ +23 hours by HPLC.

The following table shows the % purity measured by HPLC. isoA + APD(S) isoB + APD(S) isoC + APD(S) isoD + APD(S) T ₀ 95.4% 92.3% 91% 98.6% T ₀ + 23h 86% 83% 84% 92%

The loss of purity is due to the chemical degradation (hydrolysis of the amide functions) of the product due to the conditions imposed by the isomerization reaction.

As the conditions allowing the isomerization of the various compounds induce a strong chemical degradation of the products via the hydrolysis of the amide functions, the isomerization cannot be carried out in a clean and selective manner directly on the complex of formula (II) obtained according to the process described in the patent EP 1 931 673 .

Claims (14)

1. Process for preparing the complex of formula (II) below:

   

   consisting of at least 80% of a diastereoisomeric excess comprising a mixture of the II-RRR and II-SSS isomers of formulae:

   

   

   comprising the following successive steps:

   a) Complexation of the hexaacid of formula (III) below:

   

   with gadolinium to obtain the hexaacid gadolinium complex of formula (I) below:

   

   b) Isomerization by heating of the hexaacid gadolinium complex of formula (I) in an aqueous solution with a pH of between 2 and 4, to obtain a diastereoisomerically enriched complex consisting of at least 80% of a diastereoisomeric excess comprising a mixture of the I-RRR and I-SSS isomers of said hexaacid gadolinium complex of formula (I), of formulae:

   

   

   and

   c) Formation, from the diastereoisomerically enriched complex obtained in step b), of the complex of formula (II), by reaction with 3-amino-1,2-propanediol.
2. Process according to Claim 1, characterized in that the hexaacid gadolinium complex of formula (I) formed in the course of step a) is directly subjected to the isomerization step b) without being isolated or purified.
3. Process according to Claim 1 or 2, characterized in that the aqueous solution of step b) comprises acetic acid.
4. Process according to any one of Claims 1 to 3, characterized in that step b) is performed at a temperature of between 100°C and 120°C, typically for a time of between 12 hours and 48 hours.
5. Process according to any one of Claims 1 to 4, characterized in that step b) comprises the following successive steps:

   b1) Isomerization by heating the hexaacid gadolinium complex of formula (I) in an aqueous solution with a pH of between 2 and 4, to obtain a diastereoisomerically enriched complex consisting of at least 80% of the diastereoisomeric excess comprising the mixture of the I-RRR and I-SSS isomers of said hexaacid gadolinium complex of formula (I), and

   b2) Isolation by crystallization of said diastereoisomerically enriched complex, preferably by crystallization by seeding.
6. Process according to Claim 5, characterized in that the diastereoisomerically enriched complex from step b) isolated by crystallization is purified by recrystallization, to obtain a diastereoisomerically enriched and purified complex.
7. Process according to Claim 5 or 6, characterized in that the diastereoisomerically enriched complex from step b) is further enriched by selective decomplexation of the diastereoisomers of the complex of formula (I) other than the I-RRR and I-SSS diastereoisomers, i.e. by selective decomplexation of the I-RSS, I-SRR, I-RSR, I-SRS, I-RRS and I-SSR diastereoisomers.
8. Process according to any one of Claims 1 to 7, characterized in that the diastereoisomerically enriched complex obtained on conclusion of step b) according to any one of the variants described above has at least 85% of the diastereoisomeric excess comprising the mixture of the I-RRR and I-SSS isomers.
9. Process according to any one of Claims 1 to 8, characterized in that the diastereoisomerically enriched complex obtained on conclusion of step b) according to any one of the variants described above has at least 90% of the diastereoisomeric excess comprising the mixture of the I-RRR and I-SSS isomers.
10. Process according to any one of Claims 1 to 9, characterized in that step c) comprises the following successive steps:

    c1) formation of a triester of formula (VIII)

    

    in which R₁ represents a (C₁-C₆) alkyl group, notably by reaction in the alcohol of formula R₁OH in the presence of an acid such as hydrochloric acid, and

    c2) aminolysis of the triester of formula (VIII) with 3-amino-1,2-propanediol, notably in the alcohol of formula R₁OH in the presence of an acid such as hydrochloric acid.
11. Process according to Claim 10, characterized in that the triester of formula (VIII) is not isolated between steps c1) and c2).
12. Process according to Claim 10 or 11, characterized in that R₁ represents a methyl group.
13. Process according to any one of Claims 1 to 12, characterized in that step c) comprises the following successive steps:

    c1) formation of a methyl triester of formula (IV)

    

    notably by reaction in methanol in the presence of an acid such as hydrochloric acid, and

    c2) aminolysis of the methyl triester of formula (IV) with 3-amino-1,2-propanediol in methanol in the presence of an acid such as hydrochloric acid, during which the methanol is removed by vacuum distillation, until a temperature of greater than 55°C is reached, the reaction medium being maintained at this temperature under vacuum for a time typically greater than 5 hours before being cooled to room temperature and diluted with water.
14. Process according to any one of Claims 1 to 13, characterized in that the hexaacid of formula (III) as defined in Claim 1 is obtained by alkylation of pyclene of formula (V):

    

    with dibutyl 2-bromoglutarate, to obtain the butyl hexaester of formula (VI)

    

    followed by a step of hydrolysis, leading to said hexaacid of formula (III).

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